1. Field of the Invention
The present invention relates to an image forming apparatus having a detachable processing unit.
2. Description of the Related Art
An electrophotographic image forming apparatus includes an image carrying member on which an electrostatic latent image is formed. The electrostatic latent image is then developed by using a developer and transferred on a recording medium. In some image forming apparatuses, a process cartridge is detachably installed in a main body of the image forming apparatus (hereinafter, “apparatus main-body”). Such a process cartridge includes in a cartridge housing a photosensitive drum, which is an image carrying member, and one or more processing units such as a charging unit, a developing unit, and a cleaning unit arranged around the photosensitive drum.
FIG. 18 is a schematic diagram of a situation in which a process cartridge 201 is detached from a conventional apparatus main-body. FIG. 19 is a schematic diagram of a situation in which the process cartridge 201 is installed in the apparatus main-body.
The process cartridge 201 includes a photosensitive drum 202 and a developing unit 205 as a driven unit. A rear flange 202b of the photosensitive drum 202 has a rear drum-shaft hole (not shown). A concave gear 221 with a conical pitch surface is arranged on the outer surface of the rear flange 202b around the rear drum-shaft hole. A front flange 202c of the photosensitive drum 2 has a front drum-shaft hole 202e at the center. A cartridge rear plate 211 is arranged on one side and a cartridge front plate 218 is arranged on the other side of the photosensitive drum 202 along the axial direction. The photosensitive drum 202 is rotatably supported on the cartridge rear plate 211 and the cartridge front plate 218. The position of the photosensitive drum 202 is not determined when the process cartridge 201 is in a detached state. The developing unit 205 includes a developing roller 205g, a developing roller gear 258, an idler shaft 259, and a driven gear 260. The developing roller 205g is supported on the cartridge rear plate 211 and the cartridge front plate 218. The idler shaft 259 is fixed to the cartridge rear plate 211. The driven gear 260 is rotatably arranged on the idler shaft 259 and drives the developing roller 205g. 
The cartridge rear plate 211 has an engagement slot 270. A support bearing 271 is arranged on the cartridge front plate 218.
The apparatus main-body includes a main-body plate front 225 and a main-body rear plate 291. A supporting plate 289 is fixed to the main-body rear plate 291. A drum driving motor 281 is arranged on the supporting plate 289. A drum shaft 202a is rotatably fixed to the supporting plate 289 via a support bearing 290. When the process cartridge 201 is installed in the apparatus main-body, the drum shaft 202a passes through the photosensitive drum 202 in the axial direction. A coupling mechanism 293 linearly couples the drum shaft 202a with a drum motor shaft 281a of the drum driving motor 281. A first pulley 286, a convex gear 220 with a conical pitch surface, and a support bearing 215 are fixed to the drum shaft 202a. 
A cartridge driving shaft 282 is rotatably fixed to the main-body rear plate 291 and the supporting plate 289 via support bearings 284a and 284b, respectively (two-point support configuration). A second pulley 283 is fixed to the cartridge driving shaft 282. A timing belt 285 is stretched around the first pulley 286 and the second pulley 283. A driving gear 262 is fixed to a front end of the cartridge driving shaft 282 facing the process cartridge 201. A support bearing 226 is fixed to the main-body front plate 225 for supporting the front end of the drum shaft 202a. 
When the process cartridge 201 is installed in the apparatus main-body by opening the main-body front plate 225, the drum shaft 202a passes through the photosensitive drum 202 and the concave gear 221 engages with the convex gear 220 (see FIG. 19). In this way, the position of the photosensitive drum 202 with respect to the apparatus main-body is determined. At the same time, the engagement slot 270 engages with the support bearing 215 such that the position of the process cartridge 201 with respect to the apparatus main-body is also determined. Moreover, the driven gear 260 engages with the driving gear 262.
As described above, the idler shaft 259, on which the driven gear 260 is arranged for driving the developing roller 205g, is fixed to the cartridge rear plate 211 in the process cartridge 201; while the cartridge driving shaft 282, on which the driving gear 262 is arranged for driving the driven gear 260, is rotatably fixed to the main-body rear plate 291 in the apparatus main-body. Thus, if the position of the process cartridge 201 with respect to the apparatus main-body is determined based on the drum shaft 202a, accumulation of the positioning tolerance may result in distance fluctuation between the shaft centers of the idler shaft 259 and the cartridge driving shaft 282. As a result, vibrations are generated when the driven gear 260 engages with the driving gear 262 to receive the driving force. Those vibrations reach the photosensitive drum 2 and result in a traverse stripe effect in an image formed thereon.
Japanese Patent Application Laid-open No. 2004-45603 discloses a coupling mechanism for coupling a driven shaft and a driving shaft. Even if the centers of the driving shaft and the driven shaft are out of alignment, the coupling mechanism enables the transmission of a driving force from the driving shaft to the driven shaft without the occurrence of vibrations.
FIGS. 20A to 20C are explanatory diagrams of a coupling mechanism 316 disclosed in Japanese Patent Application Laid-open No. 2004-45603. As shown in FIG. 20A, a driven shaft 315 is shown uncoupled with a driving shaft 320. As shown in FIG. 20B, the driven shaft 315 is shown coupled with the driving shaft 320 via the coupling mechanism 316. FIG. 20C is a view of the coupling mechanism 316 when viewed from the driving shaft 320.
The coupling mechanism 316 includes a tubular first coupling portion 319 in which the driven shaft 315 fits and a second coupling portion 318 in which the driving shaft 320 fits. The first coupling portion 319 has an elongate guide hole W. When the driven shaft 315 enters into the first coupling portion 319, a slide pin 331 passes through the guide hole W and fits in a though hole (not shown), which is close to the front end of the driven shaft 315 facing the driving shaft 320 and in alignment with the guide hole W. In this way, the driven shaft 315 fits in the coupling mechanism 316.
A spring bearing 332 is fixed to the driven shaft 315. A coil spring 317 is arranged between the spring bearing 332 and the coupling mechanism 316 such that the coupling mechanism 316 is maintained biased towards the driven shaft 315.
An internal diameter ‘a’ of the first coupling portion 319 is bigger than a diameter ‘b’ of the driven shaft 315. Thus, the driven shaft 315 fits in the coupling mechanism 316 with a clearance distance Q therebetween. Such a configuration enables the coupling mechanism 316 to oscillate around the slide pin 331.
The second coupling portion 318 is a cup-like portion having two protruded members V that face each other and protrude towards the axis of the driving shaft 320 (see FIG. 20C). A driving pin 330 is fit in a through hole (not shown) close to the front end of the driving shaft 320 facing the driven shaft 315. The sides of the driving pin 330 protrude from the driving shaft 320 with a phase difference of 180°.
When the shaft centers of the driving shaft 320 and the driven shaft 315 are out of alignment, the driving pin 330 may not be able to enter into the second coupling portion 318. In that case, the coupling mechanism 316 oscillates around the slide pin 331 and rests in a tilted position with respect to the driven shaft 315. That enables the driving pin 330 to enter into the second coupling portion 318 and engage with a surface Va of the protruded members V. Thus, even if the shaft centers of the driving shaft 320 and the driven shaft 315 are out of alignment, the driving force is transmitted from the driving shaft 320 to the driven shaft 315 without the occurrence of vibrations. As a result, the image quality can be maintained by preventing a traverse stripe effect in an image.
Although the driving pin 330 can enter into the second coupling portion 318 when the shaft centers of the driving shaft 320 and the driven shaft 315 are out of alignment, only one side of the driving pin 330 engages at a time with one of the protruding members V (see FIG. 21A). As the driving shaft 320 rotates, a tip of the other side of the driving pin 330 engages with the other protruding member V (see FIG. 21B). As the driving shaft 320 keeps rotating, the engagement position on the protruding members V gradually shifts from the tip of the driving pin 330 towards the driving shaft 320. The circumferential speed at the tip of the driving pin 330 is higher than the circumferential speed at a position close to the driving shaft 320. Consequently, the rotating speed transmitted to the coupling mechanism 316 is higher when the tip of the driving pin 330 engages with the protruded member V (see FIG. 21A) than when a position close to the driving shaft 320 engages with the protruded member V (see FIG. 21B). As a result, the rotating speed of the developing roller 205g fluctuates thereby varying the image density in an image. The variation in the image density occurs because when the rotating speed of the developing roller 205g is low, less amount of developer is coated on the photosensitive drum 202. On the other hand, when the rotating speed of the developing roller 205g is high, more amount of developer is coated on the photosensitive drum 202.
To solve such a problem, a coupling mechanism 390 is implemented to couple the driven shaft 315 with the driving shaft 320 (see FIG. 22). The coupling mechanism 390 includes a coupling joint 391 and a tilt angle negating unit 392. The driving shaft 320 is fixed to the main-body rear plate 291 via a support bearing 292 (one-point support). In that case, the fixed position of the driving shaft 320 is maintained at more than a predetermined distance from the front end of the driving shaft 320. Such one-point support configuration allows the driving shaft 320 to easily move in a radial direction around the fixed position on the main-body rear plate 291 as compared to the two-point support configuration shown in FIG. 18.
The coupling mechanism 390 includes a driven-shaft coupling member 349, a driving-shaft coupling member 350, a leaf spring 360, and a driven-shaft fixing member 370. The driven-shaft coupling member 349 engages with the driving-shaft coupling member 350 to form the coupling joint 391. The leaf spring 360 is fixed between the driven-shaft coupling member 349 and the driven-shaft fixing member 370 by using bolts and the like to form the tilt angle negating unit 392.
When the shaft centers of the driving shaft 320 and the driven shaft 315 are out of alignment, the front end of the driving shaft 320 tilts in the radial direction and gets coupled with the driven shaft 315.
In that case, the transmission of torque through the coupled portion of the driven-shaft coupling member 349 and the driving-shaft coupling member 350 undergoes fluctuation by one rotational period. That fluctuation affects the operating speed of a driving motor (not shown), which is the driving source for the driving shaft 320, and in turn fluctuates the rotating speed of the developing roller 205g. As a result, the image density in an image varies. To avoid such a problem, it is necessary to negate the effect of the torque fluctuation. That can be achieved by using the tilt angle negating unit 392 in which the leaf spring 360 bends to negate the effect of the torque fluctuation. Thus, the driven shaft 315 rotates at a constant speed at which the driving shaft 320 rotates.
However, because the coupling joint 391 and the tilt angle negating unit 392 are arranged at different locations along the axial direction, the coupling mechanism 390 inevitably becomes larger in along the axial direction. That affects the compactness of the image forming apparatus. Moreover, the configuration becomes complicated because of the presence of the coupling joint 391 and the tilt angle negating unit 392 as separate parts. As a result, the manufacturing cost of the image forming apparatus increases.